Well one will work with any game on high, especially if you overclock it. But two will eat any game on ultra. You should consider getting three monitors if you do having a second card. When you get a chance fill out your system specs in the UserCP section.

If you like high fps on todays shooters, then 2 isn't overkill. If you settle for 40 fps max at ultra settings, then sure, 1 would be enough.
But if you can spare the money for 2 cards, then I would certainly go for it.
And who knows? Maybe you find a game you would like to play on 2 monitors? Just because you have them? And then maybe you like it, and want the second card after all.
But then that's always an option, to get a second card at a later stage.

if you have the money to spend, spend it on 2 680s, you may not need the power now but if you don't plan on upgrading later for a few years mind aswell spend it on flagships even if it is only 5% better than a 670